Chronic obstructive pulmonary disease (COPD) is emerging as an important cause of morbidity in HIV-infected patients, likely due to multiple factors, including an increased prevalence of smoking, chronic inflammation and immune activation, oxidant stress and respiratory infections. Our preliminary data suggest that several lung matrix metalloproteinases (MMPs) are upregulated in HIV-infected smokers, which could contribute to accelerated emphysema by virtue of their ability to degrade extracellular matrix and basement membrane components. Our Specific Aim is to determine the safety, tolerability, and biologic effects of twice daily doxycycline for 6 months in HIV-infected subjects with COPD. To address this aim, we will conduct a randomized, double-blind, placebo-controlled pilot study of doxycycline 100 mg twice daily in 30 HIV-infected subjects with COPD (2:1 doxy:placebo). The primary endpoint will be safety/tolerability and secondary endpoints will include change in FEV1, reduction of MMP activity in epithelial lining fluid and cells obtained by bronchoscopy and doxycycline levels in blood, ELF and bronchoalveolar lavage (BAL) cell pellets. In addition to providing novel insights into the biologic effects of doxycycline in the lung, the pilot study will inform selection of endpoints for a phase II trial, which ultimately will address an unmet medical need for novel interventions for COPD/emphysema in HIV-infected patients.